Technologies

Variable Damping Induction Coil for Metal Detection

Reference#: P01973

Pulse induction metal detector (PIMD) antennas come in two basic types. First is a single transmit and receiver coil. The second use separate transmitter and receiver coils. Two basic problems exist with existing PIMD's. First is the high kickback voltage of the transmitter coil, which temporally blinds the receiver coil from the amplifying metal target signals near the turn-off time of the transmitter coil. Second is that the protection circuitry typically has a delay time that also temporally blinds the receiver coil during the same turn-off time of the transmitter coil.

The Johns Hopkins Applied Physics Laboratory has developed and is in the process of patenting a PMID that includes a variable resistor for controlling coil characteristics for both transmitter and receiver modes of operation. A coil coupled with an electronic switch is charged with current from the power source in a transmitter mode such that when the electronic switch is abruptly switched closed the coil emits a magnetic field that induces eddy currents in a metal target. Once the transmitter magnetic fields have decayed enough, the coil can be operated in a receiver mode to detect a magnetic field resulting from the eddy currents in the metal target. The variable resistor is coupled across the coil for varying the resistance across the coil to optimize the PMID sensitivity to the metal target.